This application is related to Japanese application No. 2000-380577 filed on Dec. 14, 2000, whose priority is claimed under 35 USC xc2xa7119, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a surface acoustic wave (SAW) device. More particularly, the invention relates to a SAW device having been improved in electric power resistance. The SAW device of the invention can be preferably used, for example, as a SAW device (such as a resonator, an interstage filter and a duplexer) for a mobile communication terminal, such as a car telephone and a mobile phone.
2. Description of Related Art
In recent years, development of a mobile communication terminal, such as a car telephone and a mobile phone, of a small size and a light weight has been quickly advanced. According to the development, parts used in the mobile communication terminal are demanded to have a small size and high performance. Demands of a SAW device (such as a resonator, an interstage filter and a duplexer) contributing to the reduction of the size of a radio frequency (RF) section of the mobile communication terminal is therefore quickly extended.
An antenna duplexer, as one kind of the duplexer, is positioned at a front-end of the RF section and is required to have high electric power resistance. However, a dielectric device has been used therefore because the conventional SAW device has insufficient electric power resistance. Owing to the large size of the dielectric device, the RF section is inhibited from reduction in size.
In general, the SAW device has such a structure that contains a piezoelectric single crystal substrate, such as LiNbO3, LiTaO3 and quartz, having formed thereon interdigitated electrodes. As the interdigitated electrodes, Al (aluminum) or an Al alloy is widely used since they have excellent characteristics, i.e., a low resistance, a light weight and good property for microfabrication. When a high radio frequency signal of high electric power is applied to the SAW device, electric migration due to the radio frequency electric current and mechanical migration due to displacement of the SAW are liable to occur in Al constituting the interdigitated electrodes. It has been known that, as a result of the deterioration of the interdigitated electrodes thus caused, the characteristics of the SAW device are also deteriorated.
In order to improve the electric power resistance of the SAW device, various investigations have been conducted for the constitution of the electrodes. Among these, a technique using a Ti layer as the lowermost layer is receiving attention (described, for example, in Japanese Unexamined Patent Publication No. HEI 5(1993)-90268, Japanese Unexamined Patent Publication No. 10(1998)-93368 and International Patent Publication No. WO99/16168). According to the technique, it has been stated that the orientation of the Al or Al alloy layer is increased to suppress grain boundary diffusion of Al, and thus the migration resistance is improved, whereby the electric power resistance of the electrode film can be improved.
However, the above-mentioned technique in question has such a problem that the orientation of the Al or Al alloy layer is partially disturbed to cause scattering in electric, power resistance of the SAW devices. The grain boundary diffusion is liable to occur in the part where the orientation is disturbed, and the migration occurs from that part as the starting point. When the migration once occurs, the resistance of the part of migration is increased, and Joule""s heat is caused at that part, whereby there finally arises a possibility of meltdown of the electrode film.
The invention relates to a SAW device comprising a substrate having a piezoelectric function on a surface thereof, and an electrode film having a four-layer structure comprising, from underneath, a first layer comprising Ti, a second layer comprising Al or an Al alloy, a third layer comprising an intermetallic compound, a silicide or a CuAl2, and a fourth layer comprising Al or an Al alloy on the substrate.
These and other objects of the present application will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.